1. Field of the Invention
The present invention relates to a signal processing apparatus used for operations for range finding and to a scanning rangefinder. In particular, the present invention relates to a signal processing apparatus that is suitable for a scanning rangefinder based on a TOF (Time of Flight) method, which includes a light emitting unit to output a pulse measurement beam, a scanning unit to periodically scan the measurement beam output from the light emitting unit in a deflected manner to a measurement target space through an optical window, a light receiving unit to detect a reflected beam reflected from a measured object present in the measurement target space, and an arithmetic unit to calculate and output a distance to the measured object based on a time difference between an output time of the measurement beam and a detection time of the reflected beam.
2. Description of the Related Art
Scanning rangefinders of this type are used for vision sensors of robots or unmanned transfer vehicles, door open-close identification sensors, monitor sensors to detect presence or absence of intruders in monitoring areas, and safety sensor to detect people or objects approaching dangerous mechanical apparatuses and to safely stop the mechanical apparatuses.
Japanese Unexamined Patent Application Publication No. 2008-70159 and Japanese Unexamined Patent Application Publication No. 2007-256191, the contents of which are incorporated herein by reference in their entirety, propose a scanning rangefinder as described below.
The scanning rangefinder includes a light emitting unit to output a pulse measurement beam, a scanning unit to periodically scan the measurement beam output from the light emitting unit in a deflected manner to a measurement target space through an optical window, a light receiving unit to detect a reflected beam reflected from a measured object present in the measurement target space and output a corresponding reflection signal, a differential processing unit to differentiate the reflection signal output from the light receiving unit, and an arithmetic unit to calculate a barycentric position of a first-order-differential reflection signal based on a rise time of the first-order-differential reflection signal obtained by first order differential by the differential processing unit, to obtain a time corresponding to the barycentric position as a detection time of the reflected beam, and to calculate and output a distance to the measured object based on a time difference between an output time of the measurement beam and a detection time of the reflected beam.
When a measured object positioned in a predetermined direction is measured with the scanning rangefinder described above, semi-transparent reflection objects such as pieces of glass may exist, or objects, such as branches of trees, that are smaller than the beam of light of the measurement beam may exist between the scanning rangefinder and a measured object that is truly targeted for detection, that is, a target measured object. Further, an optical window from which the measurement beam is emitted may be unclear and contaminated.
In this case, there is a problem in that the distance to the target measured object may not be accurately calculated, since the scanning rangefinder detects not only the reflected beam reflected from the target measured object but also reflected beams reflected from semi-transparent reflection objects such as pieces of glass, branches of trees, or further from an unclear and contaminated optical window.
Therefore, considering the case of slight intensities of the reflected beam reflected from semi-transparent reflection objects such as pieces of glass or branches of trees, a conventional scanning rangefinder is configured to include a noise filter that eliminates the reflected beam in slight intensity as noise light, so as to remove the reflected beam in slight intensity from a reflected beam targeted for the distance arithmetic.
However, when the measured objects causing the noise light, such as semi-transparent reflection objects and branches of trees, exist near the target measured object, it is impossible to determine whether the reflected beam includes the noise light and to eliminate the noise light by means of a conventional noise filter for the reason that the reflected beams reflected from a plurality of measured objects are incident on the scanning rangefinder as overlapping continuous light.
Therefore, there has been a problem in that a distance calculated by the arithmetic unit based on the reflected beam including the overlapping reflected beams reflected from the plurality of measured objects does not correspond to a correct distance to the target measured object.
Moreover, there has been a problem in that, when the scanning rangefinder is used outdoors, the measurement target space is affected according to the weather. For example, there has been an apprehension in that, when a fog rises, or when snow or rain falls, part of the measurement beam reflected on the fog, snow, or rain is incident on the scanning rangefinder, so that the reflected beam including the noise light is erroneously recognized as a reflected beam reflected from the target measured object, only to calculate an erroneous distance.